carpenter: development of the oculomotor nerve. 181 



nerve. Some are directed caudad, and contribute to the formation of 

 the ventral fibre tract, as can easily be observed in vom Eath prepara- 

 tions (Plate 6, Fig. 20, trt.fbr. v.). No fibres were observed running 

 cephalad from tlie nidulus. 



The nerve, consisting of fibres densely crowded with nuclei, arises by 

 a root which is spread out in a longitudinal direction into a- fan-shaped 

 form (Fig. 20). Seen in a plane at right angles to this, the root does 

 not show this enlargement (Plate 7, Fig. 24). The trunk of the nerve 

 pursues a straight course ventrad and laterad, terminating at the side of 

 the ventral extremity of the infundibulum (wliioh is brought by the 

 cephalic flex\ire into a position ventral to the mid-brain). The distal 

 end of the oculomotor is conspicuously enlarged, so that the whole 

 nerve, seen in cross-sections of the mid-brain, may be said to have a 

 clavate form (Fig. 24, n. oc'mot.) In parasagittal sections, tlie enlarge- 

 ment has the shape of an unsymmetrical spindle, the posterior side of 

 which has the sharper curvature. This terminal swelling is the funda- 

 ment of the cihary' ganglion (gn. cil.). 



Examination of the root of the oculomotor nerve with high powers re- 

 veals conditions strongly suggesting the migration of medullary cells. 

 Figure 11 (Plate 4) represents a section through the root of tlie third 

 nerve shown in Figure 24 (Plate 7). In this preparation, which was 

 fixed in the corrosive-acetic mixture and stained with Heidenhain's iron 

 haematoxylin, the processes projecting out from the neuroblasts run to- 

 gether and blend in such a way as to lose their identities as neuraxons. 

 Again, as in Stage II, we find nuclei lying on the nerve fibres, both 

 within and witliout the brain wall. Not all of those within are the nu- 

 clei of neuroblasts, for some (as dJ) are without cytoplasmic processes, 

 and others appear to be lying peripherally on processes apparently origi- 

 nating from neuroblasts more centrally placed. These nuclei, with 

 which very little cytoplasm is connected, answer to the description of 

 the indifferent cells of Schaper, wliich are known to possess the power 

 of locomotion, since, within the boundaries of the medullary wall, they 

 pass from the region of the germinative cells, where they originate, into 

 the mantle layer. In the present instance they can be traced beyond 

 the mantle layer to the free surface of the neural tube. Indeed, now 

 and then, one can be detected in the position of cl." lying half within 

 and half without the neural wall, in the same position in which Dohrn 

 ('9l) has figured emigrating cells in the root of the developing oculo- 

 motor nerve in selachians. Although there is little evidence of cell divi- 

 sion within the nidulus, the germinative cells at the inner border of 



